Electret Condenser Microphone Preamplifier by BC650C or BC549C

Figure 1 

Here is an Electret Condenser Microphone – ECM preamplifier circuit based on ultra low noise BC650C transistors. Due to difficulty of finding BC650C, instead of them, as you can see in the schematic, you can use BC549C transistors.

The circuit is self stabilizing and sets its Q-point at approximately half of the supply voltage at the emitter of Q2 transistor so this feature allows us to get maximum output voltage swing and highest dynamic range.

The 1kΩ resistor limits the current flowing through the microphone. It must be replaced by a 2.2kΩ resistor if the supply voltage is above 12V. You can omit this component if you are using a dynamic microphone instead of electret condenser microphone.

The design provides very high overall signal to noise ratio (SNR) and low overall noise output. A 100μF decoupling capacitor is connected across the emitter resistor of Q1 so it realizes a maximum gain for this stage. The second stage based on Q2 is direct coupled to minimize phase shift effects and achieve a flat output response from 20Hz to 100kHz.

Read More:www.zen22142.zen.co.uk/Circuits/Audio/ecmmic.html